1. Field of the Invention
The present invention relates generally to an EGR system and more particularly to a control system which can appropriately meter the flow of exhaust gas to the engine at relatively high rates during heavy load operation and which appropriately reduces the supply at low load operation and/or high engine speed operation.
2. Description of the Prior Art
A known arrangement is depicted in FIG. 1 of the drawings, and is fully described in U.S. Pat. No. 4,130,093, issued Dec. 19, 1978. In that system, exhaust gas is recirculated from an exhaust conduit 1 via an EGR passage 2 to an induction manifold 3. An EGR flow control valve 4 is disposed in the passage 2 as shown, for regulating the flow through the passage. This valve includes a vacuum motor 5 which fluidly communicates with a source of induction vacuum. To control the amount of exhaust gas recirculation, a single control or correction valve (a so called VVT valve) 6 is supplied with pressure signals originating at a venturi portion 7 of the induction manifold 3 and a zone defined in the EGR passage upstream of the flow control valve 4 and downstream of a flow restriction 8.
However, this arrangement, when set to recirculate relatively large amounts of exhaust gas during high load operation when relatively large amounts of NOx are formed, has suffered from the drawback that excessive amounts of exhaust gas are recirculated at low load and/or high engine speed operation. This is of course undesirable, as the NOx formation under these conditions is relatively low, thus requiring only a low rate of EGR. Further, the excessive recirculation under such conditions leads to poor fuel economy and poor drivability.